One challenge confronting circuit designers in the electronics industry is finding more efficient and reliable methods of electrically connecting an integrated circuit chip to other circuitry. One technique is referred to as tape automated bonding, or "TAB." This interconnection technique utilizes a continuous insulative tape which is similar to photographic film to provide a planar substrate for chips that are attached to individual frames of the tape. A spider-like metal pattern of conductive traces is etched or punched from a metal foil on each frame. The traces may "fan out," i.e. radiate from the center of the frame to the edges, or may be sets of parallel lines. A chip is carefully aligned with the center of the frame, so that contacts of the chip are precisely located at corresponding conductive traces in the central portion of the frame. The chip is then attached to the TAB frame. The connection of the chip contacts to the inner portion of the frame is referred to as "inner lead bonding." The electrical performance of a single-metal TAB device is limited by parasitic inductance and capacitance of the assembly of traces. The parasitic properties limit the frequency bandwidth of the TAB device and reduce the signal-to-noise ratio.
A technique for reducing parasitic inductance and capacitance is to form a ground plane on the side of the TAB frame opposite to the conductive traces. The resulting "double-metal TAB device" is better equipped to meet the requirements of high speed digital and high frequency analog applications.
The ground plane of a double-metal TAB device must be electrically connected to establish ground potential. This can be done by the use of through holes that extend through the insulative tape from a metal foil that is used as the ground plane to ground traces of the pattern of conductive traces on the opposite side. The through holes are filled with metal. A second method of grounding the metal foil that forms the ground plane is described in U.S. Pat. No. 5,142,351 to Matta, which is assigned to the assignee of the present application. The controlled impedance TAB frame of Matta is formed without through holes. Instead, the metal foil that forms the ground plane is etched to include leads that are cantilevered from the insulative tape. The leads may then be attached to a grounded contact of the assembly to which the TAB frame is electrically connected.
While the double-metal TAB devices perform well as controlled impedance assemblies, in many circuit applications the assembly is cost prohibitive. The double-metal version of a TAB frame may have a cost that is four times that of the single-metal version. The substantial increase in cost to form a double-metal assembly applies equally to the fabrication of other controlled impedance flex circuits, such as those used as flexible printed circuit boards and flexible circuits for multi-chip modules.
What is needed is a method of forming an interconnection system for achieving a controlled impedance of a flexible circuit in a high yield and economical manner.